So, foiling makes the AC72 do 3x windspeed, allows Hydroptere to do over 50knots, makes Moths much faster, makes kiteboards faster, but on windsurfers, maybe only faster in the lightest air, or equally fast as Formula? Why not more advantage to foiling a windsurfer?

Probably because there is not enough reduction in drag between fully planing and foiling to make much of a difference. When a catamaran like an AC72 or a boat like a Moth isn't foiling, it's in displacement mode. The difference in drag between displacement sailing and foiling is huge. When a boat is sailing in displacement mode, the hull has to move an amount of water equal to the boat's weight every time it moves one boat length. Once it pops up on the foils, that all goes away and the drop in drag is huge.

A windsurfer on full plane is not bound by the rules of displacement sailing and only a very small amount of board area is in contact with the water, so very little drag to begin with. Riding on a foil probably has the potential to be slightly lower drag than riding on a flat planing surface, but the downside is a craft which is much more difficult to control. In order to make the foil controllable, all the foiling windsurfers I've seen have tabs and extra control surfaces which add more drag (looks like a stingray stuck on the bottom of the fin). So a planing windsurfer already has the advantage of a low-drag and very controllable hull. Another downside to foiling on a windsurfer is that as the board lifts out of the water, less of the fin is in contact with the water, so leeway angle would increase which lowers speed. The end plate effect between the bottom of the board and the fin is also lost.

I'm sure if even a fraction of the AC72 budget was put towards developing foiling windsurfers, there would be some advancements made, but at this point the development just isn't there to make any significant improvements over a fully planing windsurfer.

I'm not a physics guy so I can't give any technical explanations, but after watching and sailing with a kite foil close hand in marginal conditions I think part of the reason a kite foil works better than a windsurfing foil is getting underway. The kite boarder can fly or direct his kite overhead and get immediate lift so he is riding the foil even before planing. The kite board was being pulled up than forward on his foil. A windsurfer would have to schlog first & build up enough speed to get up on the foil which would take more time, effort & probably skill than with the kite.

The huge advantage I see on the kite foil is in up wind down wind sailing in choppy conditions. The kite foil just glides through like it is on flat water. I'm guessing a windsurfing foil would be more efficient going up/down wind as well rather than on a beam reach. The other advantage of the kite foil is in changing directions as they don't have to worry about flipping sail or changing feet. I can't imagine trying to jibe a windsurfing foil.

Both you guys are correct, based on my observations.
I rode RichMiller's double wing foil windsurf board, and while starting out, tacking, and going in a straight line were pretty basic, the jibe did take a drawn out balancing act. Upwind and downwind were advantages, pure boat speed not, as a planing windsurf board goes across the chop, not into or away from the windswells.
I haven't kited since '01, but in PuertoRico, while 5 Formula guys watched from the seawall behind me, we watched a foil kiteboarder go from PuntaLasMarias down 3 miles to Condato, and back, in around 47 minutes. I wasn't impressed, I don't course slalom. But all 5 Formula guys were.

Also not a fluid dynamics guy, but I'm guessing top foil craft speeds are more or less the result of the ratio between sail area and vessel weight. If you can make this ratio high you can use a smaller foil (less drag) to create lift. For windsurfers the rider weight means we need a pretty big draggy foil to create lift in comparison to the relatively tiny foils that the ac72s can use.

you may have possibly overlooked "design speed" as a factor there. the AC's seem to be optimized between 12~16 knots. say at 17 knots they are moving right along, catching a chance gust puts them well into the 50's. once it starts getting into the 20's their speed drops noticeably back down into the 40's. there's no increase in water resistance so what is with that?? i'm not so sure their foils are 'relatively' smaller than say a formula fin and the tiny, intermittent contact patch of board

Probably because there is not enough reduction in drag between fully planing and foiling to make much of a difference.

I was also inspired by watching the Americas Cup to do some web research on foil windsurfing. It looks like AHD is trying to market a foil board with a $7000 price tag. In the right conditions there are definite advantages, but overall not worth the extra effort and expense for me.

Watching the guys riding the foiling boards, it just doesn't look like they're able to get into a really locked in, powered up stance. They are standing over the board. I think that is probably really limiting their speed. If you look at the placement of the fin on the foil boards, its much farther forward than it would normally be because the center of lift of the foil has to be under the CG of the board. With the fin placed forward, you can't drive off the back foot in a locked in stance as you would with a "normal" board. Also, with the board riding so high, most of the fin is out of the water meaning not much lateral resistance to drive against. Maybe they should put the lifting foils in the middle of the fin rather than all the way at the bottom. Anyway, I think these would be some main contributing factors as to why a foiling board is not much faster (or even slower) than an equivalent slalom board. Plus the lifting foil, stabilizing foil and connecting fuselage/bulb are probably equal or greater drag than a small flat planing surface. It's just a lot of "stuff" to be dragging through the water.

It definitely looks cool though, and I'm really impressed at the amount of engineering that went into the assembly, really slick.

I was thinking that a very small foil, more of a winglet, at the tip of a regular fin might give just enough extra lift to the tail without adding to the drag and be enough to create a phase between planing and foiling just above the surface of the water.
I just need to find someone to build the prototype.

I was thinking that a very small foil, more of a winglet, at the tip of a regular fin might give just enough extra lift to the tail without adding to the drag and be enough to create a phase between planing and foiling just above the surface of the water.

Problem is that a "regular" fin sits behind the center of gravity of the board, behind your feet. If you just stick a lifting foil on the tip of a fin, I suspect the result will be that the tail will lift and the nose will be driven down (or even more likely, you'll just create a bunch of drag). The center of lift has to be farther forward, somewhere between the rider's feet would be my guess. That way you can control the pitch of the board by weighting your front or back foot to keep everything in trim.

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